1. Field of the Invention
The present invention relates to a link chain such as a silent chain and a leaf chain in which a plurality of link plates is linked by pins and more specifically to a link chain in which bushes are fitted into the link plates turning relatively with the pins.
2. Related Art
Generally, there is a silent chain that is composed of endlessly linked toothed link plates by pins and is used as a chain for transmitting power and a leaf chain that is composed of link plates having no teeth and linked straightly by pins and is used as a chain for lifting or drawing a heavy load.
Lately, the applicant of the present invention has proposed a link chain composed of pins on the surface of which a metal carbide layer formed of at least one of chrome, vanadium, niobium, titanium, zirconium, tantalum, molybdenum and tungsten is provided by means of cementation and having Vickers hardness of 1300 Hv or more is coated by 5 μm or more and link plates formed of carbon steel or alloy steel containing carbon in a range of 0.30 to 0.55 wt % or as a base material and on the surface of which a metal carbide layer formed of at least one of chrome, vanadium, niobium, titanium, zirconium, tantalum, molybdenum and tungsten is provided by means of cementation and having Vickers hardness of 1300 Hv or more is coated by 3 μm or more (see Japanese Patent Application Laid-open No. 2000-249196 Gazette).
There has also been devised a silent chain in which bushes are fitted into link plates that turn relatively with pins (see Japanese Patent Application Laid-open Nos. Hei. 7-151192 and 2002-250405 Gazettes).
While it is possible to reduce wear of pins and pinholes and to reduce elongation of the chain because the surface of the link plates of the silent chain of JPA No. 2000-249196 is coated with the metal carbide layer whose hardness is high similarly to the surface of the pins, there is a drawback that because the whole surface of the link plate is hardened, the surface of teeth of a sprocket and a tensioner (including a tightener and a guide shoe) that slidably contact with the link plates are worn away sooner.
Due to that, while it is also proposed to remove the surface of metal carbide layer of the link plate engaging with the surface of the teeth of the sprocket and/or the part thereof slidably contacting with the tensioner, a long processing time of 50 to 100 hours is required to remove the metal carbide layer of the engaging tooth flank and the back surface of the link plate by barreling. Still more, because corners of the link plate are also polished and rounded because the whole outer side surface is polished, an effective flat surface of an edge of the link plate that engages with the sprocket or slidably contacts with the tensioner is considerably reduced. Due to that, there is a possibility that it may cause quick wear and noise of the sprocket and the tensioner and may generate pitching (micro-fatigue spalling) of the surface of teeth of the sprocket.
Although it is possible to remove the metal carbide layer by grinding the surface of the link plate engaging with the sprocket or the back thereof, it is required to precisely control the grinding size of all of the link plates to keep precise engagement of the large number of link plates with the surface of the teeth of the sprocket in this case, thus considerably increasing an associated cost.
Still more, while a large amount of diffusion metalizing material, e.g., metal powder of chrome, to coat the metal carbide layer over the whole link plate, these carbide-forming metals are expensive and increase associated costs.
Further, while punching by a press is carried out in forming the pin holes through the link plate, the whole hole surface does not become a sheared surface and part thereof becomes a fracture surface. As a consequence, a diameter of the pin hole on the side of the sheared surface becomes different from that on the side of the fracture surface and a contact area of the pin hole with the pin becomes small, so as to provide a cause of the quick elongation of the chain.
Meanwhile, while the silent chain in which bushes are fitted into bush (pin) holes of link plates is also known as shown in Japanese Patent Application Laid-open No. Hei. 7-151192, the bush is formed by rolling a steel plate and after rolling, carburizing, quenching and tempering are carried out to harden the surface. However, strain is prone to be generated at a seaming part of the bush in rolling and it is difficult to obtain enough roundness. Still more, even if the hardness of the bush described above is adequate for the carburized and quenched pin, the hardness of the carburized and quenched bush is not enough for the pin on which the metal carbide layer is coated. Such bush wears relatively quickly and may cause elongation of the chain.
Still more, although a link chain in which a bush is formed as a solid type bush having no seam in the longitudinal direction is also proposed as disclosed in Japanese Patent Application Laid-open No. 2002-250405, the bush is also formed of alloy case-hardening steel and does not have enough hardness relative to the pin coated with the metal carbide. It is noted that a part of the link plate between the bush (pin) hole and the outer edge surface is prone to be a fracture surface by receiving a large shearing force from the pin in the silent chain and due to that, the silent chain of Japanese Patent Application Laid-open No. 2002-250405 links two link plates by a solid bush to assure predetermined strength.
Further, although it is necessary to thin the bush and to assure a predetermined distance between the bush hole and the outer edge surface of the link plate to keep a predetermined tensile strength of the link plate in fitting the bush into one link plate, it is considered to be difficult to carry out deep drawing for forming the thin bush by the carbon steel or alloy steel containing carbon in high concentration of 0.30 to 0.55 wt % that is required to generate the metal carbide layer in general.